1. Field of the Invention
The subject of the invention is an optical filtering/electromagnetic screening structure for being joined to at least one transparent substrate, especially made of glass, the structure comprising at least two plastic sheets and including, or intended to be joined to the sheets, a conducting electromagnetic screening element.
The invention will more particularly be described with regard to the use of such an electromagnetic screening structure for a display screen, such as a plasma screen.
2. Description of the Background
A plasma screen includes a plasma gas mixture (Ne, Xe, Ar), trapped between two glass sheets and phosphors placed on the internal face of the rear sheet of the screen. Ultraviolet light radiation emitted by the plasma gas mixture during plasma discharge between the two glass sheets interacts with the phosphors on the internal face of the rear sheet in order to produce the visible light radiation (red, green or blue). In competition with the UV emission is a gas particle de-excitation mechanism that generates infrared radiation between 800 and 1250 nm, the propagation of which, mainly through the front face of the screen, may be the cause of very substantial disturbance, especially for nearby infrared-controlled, for example, remote-controlled, equipment.
Apart from the infrared radiation generated by de-excitation of the plasma gas, intense radiation in the orange at 590 nm is also emitted by the gas mixture when the latter contains neon. This radiation in the orange may, to the viewer's eyes, be unattractive. Furthermore, it interacts with the blue and green colours of the light spectrum so that it renders an image on the display screen having blue and green colours that can be termed washed-out or faded, and a less sharp red colour.
Moreover, like all electronic appliances, plasma screens have addressing systems (drivers) that may generate parasitic radiation with respect to other devices with which they must not interfere, such as microcomputers, portable telephones, etc.
To eliminate, or at the very least reduce, the propagation of this radiation, one solution consists in placing a structure that is both transparent and metallized against the front face of the screen in order to provide electromagnetic screening.
One known type of structure consists of two sheets of a thermoplastic, particularly PVB, between which an array of metal wires in the form of a homogeneous grid is placed.
This grid may be formed from a metal wire gauze adhesively bonded between the two PVB sheets by heating the thermoplastic.
Another solution consists in etching a copper layer deposited on a transparent substrate, such as PET, using a standard photolithographic technique, and joining this substrate to the PVB sheets, the PET substrate being inserted between the two PVB sheets and bonded by heating the thermoplastic.
This advantageously laminated structure, with the PVB being placed on the front of the display screen, the front face of which is a glass substrate, protects the viewer should the screen fracture, by retaining the glass fragments.
The metal gauze therefore provides electromagnetic screening. However, it is always desirable to improve the performance of an electromagnetic screening filter by further reducing the transmission of radiation in the infrared and to find other filtering properties therein, such as the ability to cut out a substantial amount of the orange colour. Furthermore, it is of course desirable for the colour rendering as perceived by the viewer on the screen to be optimum.
A known type of filter for motor-vehicle windows that cuts out certain wavelengths is a combination of two PVB sheets, one of which consists of 3M's product called “Solar Reflective Film” and the other of which consists of a Sekisui S-Lec® PVB film sold by Sekisui. The 3M product and Sekisui PVB film are transparent and neutral in transmission, the Sekisui film furthermore containing conducting particles. This filter has the advantage of transmitting only 51% of the light, but this light is predominately in the yellow, the dominant wavelength of which is 561 nm, which significantly modifies the colour rendering on a display screen. The light also has a purity of 8%, which is regarded as significant compared with a desired level of less than or equal to 7%. It will be recalled that the purity is defined in the 1931 CIE measurement system. In this system, the colour of an object is represented by a point with coordinates x, y. Purity is the ratio of the length of the segments, of which one joins the illuminant to the end of the spectrum locus and passes through the coordinate point x, y and the other of which joins the illuminate to the coordinate point x, y respectively. Furthermore, although this filter makes it possible to obtain transmission in the infrared at 1200 nm of at most 5%, the transmission still remains 13% at 850 nm. Finally, the orange colour is not cut out sufficiently, the transmission for the corresponding wavelength being 64%.